This invention relates to refrigeration systems, and more particularly to refrigeration systems used in household refrigerators and freezers.
Refrigeration systems for household refrigerators and freezers have heretofore been designed for low cost and high reliability, both of which require a simplicity of design, together with a minimum number of parts. Typical refrigerators or freezers employ a vapor-compression system having a fractional horsepower, electric motor driven, hermetic compressor connected in a circuit with a condenser, an evaporator, an optional accumulator, and a refrigerant flow restriction between the condenser and the evaporator. For reasons of obtaining high energy efficiency, it is desirable to utilize a relatively high duty cycle for the compressor run time, while maintaining a sufficient reserve for high ambient temperature conditions. Thus, a thermostat responsive to the temperature in the cooled cabinet is used to cycle the compressor as necessary to maintain the preselected temperature. Based on normal room temperatures and the absorption of heat into the cooled space through the insulation, the compressor duty cycle may run fifty percent to sixty percent, leaving a reserve but requiring continuous operation under very high ambient temperatures or frequent opening of the door for access to the interior of the cooled cabinet.
The flow restriction has been almost universally a capillary tube sized for optimal efficiency at a single set of conditions of ambient and internal cabinet temperature. Capillary tubes used as the sole restriction offer the advantages of low cost, high reliability, and the added efficiency of being easily placed in heat exchange relationship with the return line from the evaporator to the compressor.
The capillary tube system, which runs constantly at a single ambient temperature and constant load condition, is very efficient when the capillary tube is sized for these conditions. When this is done and the system is operating under equilibrium conditions, the refrigerant at the condenser outlet where it enters the capillary tube is a saturated or slightly subcooled liquid. This liquefied refrigerant flows through the capillary tube and undergoes a substantial pressure reduction until it enters the evaporator, where it is vaporized to absorb heat from the interior of the refrigerator or freezer.
Because the refrigerant flows in a closed system, and the actual rate of flow through the capillary tube is dependent upon the pressure differential between the pressures in the condenser and the evaporator, any change in load conditions will affect the operation of the system. In the case of refrigerators and freezers, the changes in operating conditions can result from changes in the room ambient temperature, which affects the heat dissipation from the condenser, as well as the internal conditions, which may be determined by the opening and closing of the door and the addition of warm items to affect the load on the evaporator. Furthermore, because the system must operate on a cyclic basis to maintain reserve capacity for extreme conditions, a thermostat inside the refrigerator causes the compressor to cycle on and off, and when the compressor is off, the pressure tends to equalize throughout the system, resulting in the elimination of liquid refrigerant in the capillary tube, which then becomes entirely filled with gas. The result of these changes in operating condition is that the refrigeration system is often operating under conditions other than optimum with regard to the temperatures and pressures in the condenser and the evaporator, causing a loss of energy efficiency in the system.
Some of these effects can be minimized in various ways. For example, to minimize the formation of flash gas in the capillary tube, which would tend to reduce the capacity of the system, the tube is usually soldered or otherwise placed in heat transfer relationship with the return line from the evaporator to the compressor. Because the common optimum conditions are such where the system operates at say a fifty percent duty cycle, the capillary tube is usually sized "loose" or with a reduced restriction which allows fast flooding of the evaporator during start-up and fast equalization of suction and discharge pressure during the OFF portion of the cycle.
The fast flooding of the evaporator allows the system to quickly reach a high running efficiency, thereby reducing the total compressor run time for the ON cycle. Once the evaporator is flooded, however, this type of system tends to allow gas to enter the capillary tube and pass directly into the evaporator. When gas passes from the condenser to the evaporator, it never goes through the phase change to a liquid and back to gas that is necessary to produce effective cooling in the evaporator. Not only does this load the compressor with an increased mass flow that does not refrigerate, but it also transports heat into the evaporator, to thereby reduce the efficiency of the system. When the compressor is turned off at the end of the run cycle, the pressure equalizes between the condenser and the evaporator across the capillary tube relatively quickly, and this allows hot gas and liquid to pass into the evaporator. This adds heat to the evaporator and decreases overall system efficiency. The fast equalization, however, allows a lower cost, split phase compressor motor, with its relatively low starting torque, to restart after short OFF cycle.
On the other hand, if the system uses a "tight" or more restrictive capillary tube, the system will tend to have a slightly greater efficiency during steady state run conditions, but the evaporator floods so slowly during start-up that the advantage in efficiency may be lost over the entire run cycle. Furthermore, equalization may take so long that the compressor may have starting difficulties with a short OFF cycle because the low starting torque is unable to overcome the remaining back pressure in the condenser.
In larger refrigeration systems, these problems are overcome by using a controlled expansion valve as the restriction instead of the capillary tube. Valves of this type generally use a diaphragm or bellows operated by a refrigerant system and opens or closes the valve located at the evaporator inlet to vary the amount of restriction at this point. For example, Owens U.S. Pat. No. 3,367,130 discloses an expansion valve which opens and closes in response to the amount of subcooling of the refrigerant leaving the condenser by responding to a sensor attached to the external surface of the tube at that point, which as disclosed is remote from the valve itself. However, valves of this type are too large and much too expensive to be substituted for a capillary tube in small refrigeration systems.
In the completely different area of refrigeration for automotive air conditioning, it has been proposed to provide a subcooling flow control valve to control refrigerant flow to the evaporator in conjunction with an additional downstream flow restrictor, such as an orifice. European Patent Publication No. 255,035, published Feb. 3, 1988, shows a flow control valve with an external bulb used in an automotive air conditioner with a downstream restriction that may be a capillary or an orifice. U.S. Pat. Nos. 4,788,828 and 4,840,038, both in the name of Motoharu Sato, both disclose control valves using an internal sealed bellows filled with a refrigerant for controlling flow to a downstream restriction in an automotive air conditioning system. The first of these patents shows a bleed passage bypassing the valve to allow equalization when the compressor is turned off. The second patent uses a second bellows downstream of the valve to expand and force open the valve closed by the first bellows, to allow equalization to take place across the valve. All of these arrangements are intended for automotive air conditioning where the engine provides sufficient power for the compressor under all conditions, and the purpose of the valve is to regulate flow under a wide range of flow rates resulting from widely varying engine, and hence compressor, speeds.